The actuators or actuation mechanisms to transfer mechanical power from one input to an output, existing on the market, generally include a floating endless system, resting on its ends by elastic elements and a crown that, upon turning, when it comes up against an obstacle or reaches the end of its run, the worm moves cutting off the current from the motor. The actuation mechanisms have a series of inconveniences that will be stated hereinafter, determined by the basic concept of embodiment that they apply and which have been tried to be solved in very different ways, though without attacking said basic concept of embodiment.
As it is specified in Siemen's German patent PCT/DE91/00600, upon sizing an action regulator (actuator) for valves it is necessary to take into account certain factors. On the one hand, due to defects, aging or defective maintenance (such as corrosion of the spindle, wear or excess tightening of the packing etc.) the turning torque needed to close the valve may increase considerably (for example from 30 to 80 Nm.) On the other hand, upon sizing the action regulator (actuator) said value (80 Nm) must be taken into account in unfavorable operating conditions of the actuator itself (a voltage drop in the line, tolerances, temperature of the motor, etc.) In this way the actuator in oppositely unfavorable conditions, there may be a supply of 10 times more theoretical turning torque than that which is needed (30 to 300 Nm), in the event that the dependent torque disconnection were to fail. This may seriously affect the actuated mechanism (valve, in this case), even managing to make it unserviceable.
The solution provided by this German patent is that of a breaking frame of the spindle of the valve that acts from a given maximum torque, preventing this to reach the extreme of 300 Nm in the event of failure of the control switches. With this solution the effect is reduced but the problem that causes said effect is not solved.
In order to try to overcome this effect (aside from others), actuator manufacturers include an additional microswitch, of an (intentionally) synchronized path (path limiter) with the torque limiter. Said synchronization disappears as soon as there are minimal temperature variations, once again posing the problem of destroying the valve.
In the event that there is no failure in the control s witches, these actuators have two inconveniences that can lead, at average and high output speeds (100 to 300 nm/min., according to the type of valve), to reach excess output torque values much higher than the values pre-set by said control switches. This excess output torque likewise manages to cause significant damage to the actuated valve.
As is stated in "Bulleltin FC-77 of Limitorque Corp.", said inconveniences are:
1) It does not the release of inertia from the mechanism. In this way excess pressure in the seat of the valve and excess stresses on the rod are created. These anomalies are increased up to significant values when the output speed of the actuator increases. This all makes it necessary to limit the output speeds of actuators. For example in all-or-nothing (closed loop) valves the speeds of the rod are limited to 300 mm/min for gate values and to 100 mm/min for seated valves, running the risk of damaging the valve, despite said limitations. At times, it is even essential to equip the actuator with a second high speed motor, so that in the cases of a serious emergency, the output speed increases considerably, even knowingly that this will lead to the destruction of the seat and greater damage to the value.
In adjustment and shutoff valves, the need of very powerful motors for the shutting itself, involves an increase of the inertia, which requires a reduction of the speed to maintain the precision of the adjustment, sacrificing the response speed, even in some cases, requiring the need of two valves, an adjustment valve and a shutoff valve, for said tasks.
2) These proceblems of excess pressures are increased upon additionally considering the electric signal delay between 20 and 50 mn; that exists between the signal of the switch of the torque limiter and the energization of the of the motor operating and stopping contactors. Besides this anomaly is inevitable and does not depend on the output speed of ther actuator but rather on the electric system used. Its effect simply (increase of torque above the adjusted one) is significant at average and high actuator output speeds (100 to 300 mm/min.).
These two anomalies are produced upon the motor and the other elements of the actuator being permanently mechanically linked up to the output, either by endless-crown embodiment or else in other embodiments used less but also with the same principles. Within all of them the main problem is that of collecting the excess power from the instant from the jump of the torque limitation and to dissipate it in a sure manner and without effecting the limited torque.
Within these embodiments is Bernard's French patent FR 2699983-A1 that uses a planetary reducing mechanism to limit, in its outside crown and by means of adjustable springs, the transmitted torque.
Despite the novelty of this actioning against the "classic" endless-crown, it does not solve its problems. The inertias accumulated at high speeds are still not released and the signal delay times keep affecting it in the same way.
In the moment that the torque control switch is actuated, previously adjusted to a maximum necessary torque, the signaly delay and the inertia (if they are important) make the regulating springs continue to tighten accumalatively and thus a torque greater than the adjusted one to the output shaft of the actuator is transmitted.
Practically the same solutions, when limiting the transmitted torque, are applied by German patent DE-A-3010019.
U.S. Pat. No. 3,921,264 also deals with a mechanism formed by two planetary reducers. In one of them the outside crown remains fixed by means of a solenoid that comparably energizes the motor. When the motor disconnects, the currect stops reaching said solenoid and, therefore, it totally releases the crown or cam of the planetary element. In this way no torque is transmitted by means of the planetary reducer as of this moment.
This embodiment does not solve the problem of electric delay but rather it even makes it worse by using an electrically excited solenoid. Once this delay has taken place if the torque transmission is totally disconnected, the problem of accumulated inertia is avoided. At any rate this would only serve to "absorb" accumulated inertia of the limitating value since, on the contrary, the outside crown would be kept turning with too much accumulated kinetic energy and the electric signal delay would be even greater than the normal one with the problems that this involves.
Another inconveniences and anomalies, other than the above cited ones, that, said actuators generally have are:
3) Needing an additional manual engaging-disengaging lever to carry out the same functions. In this group Japanese patent JL-60 018 679 that needs a lever that turns its coupling-decoupling elements would be included.
4) The operator needs to use both hands together, having to feel out the above cited lever with one hand, while turning the flywheel with the other hand trying to fit the spaces of the same with the teeth of the intermediate wheel, after having deblocked the motor.
After having fit the teeth in the spaces, it is then possible to operate the flywheel.
The flywheel is likewise used to tune up the path limiter of the actuator mechanism with which the mechanical power is transferred from an input to an output.
This path limitation needs great precision since in the event that the microswitch of the torque limiting mechanism theoretically becomes unservicable, the microswitch of the path limiting mechanism is the one responsible for disconnecting the motor to prevent the deterioration of the valve and/or of the actuator mechanism itself.
5) There is no mechanical manual nor automatic blocking. In the event that this were installed, in conventional actuators, it could not be used because of the permanent mechanical link, already described above, between the motor and the remaining components of the actuator.
6) Stresses higher than those produced by the maximum torque cannot be obtained, without stopping the motor, as there is no centrifugal clutch or multiplier.
7) There is no irreversibility mechanism. This irreversibility, in most motorized valves is ensured by the rod and/or nut of the valve (or of the actuator mechanism) that tends to be irreversible and with a low mechanical efficiency.
This limits the overall efficiency of the unit and prevents use of more efficient rods and nuts (longer pitches and ball spindles) that, by definition would be reversible.
8) The need of costly preparation times of the valve and of the actuator mechanism to be able to make timely checks of the stresses in the spindle.
9) Likewise, these costly preparation times are needed to be able to check inside the actuator.
10) The need to sacrifice the high speed to obtain great toroques at the output of the actuator, or else, the need to use a two speed motor or one with an electric brake.
11) The sporadic and cyclic presence of an operator to retighten the packing, by hand and to control these tightening pressures; being able to destroy the packing by exceeding its limit tightening pressure, being able to even ,at times, brake the movement of the spindle, increasing in these cases the passive stresses that would regulate the correct operation of the actuator-valve unit preventing the correct closing of the valve and thus its wear and tear.
12) Having a regulated maximum torque that is independent of the turning speed of the mechanism.